Some conventional vehicle lighting units for use in a vehicle headlamp utilize a semiconductor light emitting device as a light source. Examples of such a semiconductor light emitting device include a light emitting diode (LED), a semiconductor laser, and the like. Recently, as the optical performance of such an LED is improved, the use of the semiconductor light emitting device like an LED as a light source for such a vehicle headlamp is increasing. Hereinafter, such a vehicle lighting unit will be described with taking a headlamp as an example.
In general, this type of a vehicle headlamp can include a semiconductor light emitting device, a drive circuit configured to drive and control the semiconductor light emitting device, and a heat dissipation member configured to dissipate heat generated by the semiconductor light emitting device and the drive circuit. The drive circuit may be provided outside the lighting unit so as to be exposed in terms of the improvement in the heat dissipation efficiency and the maintenance and/or replacement workability.
Specifically, a reference is made to Japanese Patent No. 4523055, which describes a vehicle headlamp as shown in FIG. 1. As shown in FIG. 1, the vehicle headlamp can be composed of a light source module 80, a reflector 84 provided for cover over the light source module 80, a seat 85 on which the light source module 80 is mounted, a projector lens 86, and the like. The light source module 80 can include an LED 81, a drive circuit 82, and a metal substrate 83 on which the LED 81 and the drive circuit 82 are mounted so that they are integrated to constitute the light source module 80 as a unit. The drive circuit 82 can be exposed outside the reflector 84 that covers the LED 81 from above. A heat dissipation member such as a heat sink (not illustrated) can be attached to the seat 85, and the metal substrate 83 of the light source module 80 can be secured to the seat 85 so that heat from the metal substrate 83 can be conducted to the heat dissipation member. With this configuration, the heat generated by the LED 81 and the drive circuit 82 when the lighting unit is operated can be conducted to the heat dissipation member via the metal substrate 83 and the seat 85, thereby enhancing the effective dissipation of heat through the heat dissipation member.
The vehicle headlamp described in Japanese Patent No. 4523055 can have the structure in which the LED 81 and the drive circuit 82 are mounted on the metal substrate 83, and the generated heat can be dissipated through the metal substrate 83. In this structure, however, the heat from the LED 81 in a relatively larger amount may be conducted to the drive circuit 82 through the metal substrate 83 even when the drive circuit 82 also functions as a heat generation member. Specifically, the effective dissipation of heat generated by the LED 81 cannot be promoted while the temperature rise of the drive circuit 82 inevitably occurs. As a result, there is a possibility that the lifetime of the drive circuit 82 and/or the reliability thereof may deteriorate.
In some cases, an insulation grease or other coatings can be interposed between the metal substrate 83 and the seat 85 in order to enhance the insulation property therebetween. In this case, a heat-transfer resistance between the metal substrate 83 and the seat 85, which are parts of the heat dissipation path, can be increased by the intervention of such a coating. Due to the remarkable deterioration of the effect of the heat dissipation path, the heat from the LED 81 can be further conducted to the drive circuit 82, thereby causing the temperature rise of the circuit 82 to occur with ease.
Furthermore, the vehicle headlamp described in Japanese Patent No. 4523055 is configured such that the light source module 80 is fixed onto the seat 85 having a heat dissipation member (not-illustrated) in order to promote the heat dissipation of the LED 81 and the drive circuit 82. The structure as described above may hinder easy replacement of the LED 81 in the light source module 80 from the rear side of the headlamp as compared to the lighting unit utilizing, for example, a bulb as a light source. This means that maintenance workability may deteriorate.